Yes, if I were a little boy like David and ran into, say Andre the Giant and he wanted to kill me, I'd certainly believe he was Satan sent.
The bible is a very accurate history book, a continuous document of life in the Middle East in ancient times. It is one of the only complete (?) records in existence. I'm taking a course on the Old Testament right now I purchased from Great Courses. According to the accepted history, the book of Genesis and some of the other early stories and legends were transcribed circa 900 BC, from a much older oral history.
Perhaps the wildest books covering the Nephilim is the Book of Enoch (5 books total)- which is not included in the accepted bible. The Enoch epics were lost and rediscovered in the early 1800's. A case of the Catholic Church deciding what was best for us. There was as much excluded as there was given to us illiterate masses.
Book Of Enoch with YAHWEH's Name Restored | YAHWEH's Sword Download
I find it interesting that scholars tend to refer to the original Jewish faith as being a cult. Many of the the legends were adopted from other older cultures. I.e. Greek mythology, etc. The early tribes were "well traveled". We wonder what was lost to us? The Library of Alexandria, we should mourn its' loss.
Many of the early biblical figures have been thought to be mythical. Even King David's existence is questioned. Little bits of archeology discovered here and there (and some very recently) tend to indicate that he was an actual man.
If you have a couple hours to kill, Trey Smith is on Youtube with a number of extremely interesting videos. This guy is wild! Here's a link to his Nephilim story, and it is 2 hours long.